Beta-casein is the major milk protein synthesized by the mammary gland. The expression of the beta-casein gene undergoes marked changes as a function of reproductive stages. Its expression is low in virgin and postlactating states, whereas the gene is fully expressed in the lactating state. During pregnancy, beta-casein gene expression increases only to a limited extent partly because it is suppressed by progesterone, whose circulating level is increased during this period. Studies using mammary cell culture system have shown that casein gene expression can be induced by prolactin, glucocorticoid, and insulin, but is inhibited by progesterone. Thus, the beta-casein gene in the mammary gland provides a useful model for studying the molecular mechanisms of hormonal control of gene expression during development. We have studies the sequence elements responsible for beta-casein gene expression by transfection of chimeric beta-casein gene into primary mammary epithelial cells. These studies have showed that approximately 500 base pairs of the 5'-flanking sequence of the mouse beta-casein is sufficient for tissue-specific and hormonally induced expression. More recently we examined the sequence-specific binding of mammary nuclear proteins to the promoter region of the beta- casein gene by DNase I footprinting analysis and gel shift assay. These results suggested that multiple cis-acting DNA elements and their binding factors are required for hormonal induction of mouse beta-casein gene expression. In addition, our results revealed the presence of a pregnancy-specific mammary nuclear factor that binds to two separate sites of the beta-casein promoter and indicated that it may serve as a repressor that mediates the inhibitory action of progesterone on beta-casein gene transcription.